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<a name="boost.proto.external_transforms"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Struct template external_transforms</span></h2>
<p>boost::proto::external_transforms — A map from grammars to transforms, used as a way to externally associate transforms.</p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../../proto/reference.html#header.boost.proto.transform.when_hpp" title="Header &lt;boost/proto/transform/when.hpp&gt;">boost/proto/transform/when.hpp</a>&gt;

</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span><span class="special">...</span> When<span class="special">&gt;</span> 
<span class="keyword">struct</span> <a class="link" href="external_transforms.html" title="Struct template external_transforms">external_transforms</a> <span class="special">{</span>
  <span class="comment">// types</span>
  <span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">to_mpl_pair</span><span class="special">&lt;</span> <span class="identifier">When</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span><span class="special">...</span> <span class="special">&gt;</span> <a name="boost.proto.external_transforms.map_type"></a><span class="identifier">map_type</span><span class="special">;</span>  <span class="comment">// For exposition only.</span>

  <span class="comment">// member classes/structs/unions</span>
  <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Grammar<span class="special">&gt;</span> 
  <span class="keyword">struct</span> <a class="link" href="external_transforms/when.html" title="Struct template when">when</a> <span class="special">:</span>
    <span class="keyword"></span> <a class="link" href="otherwise.html" title="Struct template otherwise">proto::otherwise</a>&lt; typename mpl::at&lt; map_type, Grammar &gt;::type &gt;
  <span class="special">{</span>
  <span class="special">}</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id-1.3.34.5.54.13.4"></a><h2>Description</h2>
<p>
            It is sometimes desirable to define a grammar that can be customized with different sets of transforms.
            To do that, where you would normally specify a transform within a grammar, you can instead put
            <code class="computeroutput"><a class="link" href="external_transform.html" title="Struct external_transform">proto::external_transform</a></code>; for example:
            <code class="computeroutput">proto::when&lt; some_grammar, proto::external_transform &gt;</code>. Then, when
            invoking the grammar, you can pass an approriately-defined instance of <code class="computeroutput">proto::external_transforms</code>
            as the Data parameter. When an expression matches <code class="computeroutput">some_grammar</code>, Proto
            will look up the approprite transform in the Data parameter using <code class="computeroutput">some_grammar</code>
            as a key.
          </p>
<p>
            </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">int_terminal</span>
  <span class="special">:</span> <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">char_terminal</span>
  <span class="special">:</span> <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">my_grammar</span>
  <span class="special">:</span> <a class="link" href="or_.html" title="Struct template or_">proto::or_</a><span class="special">&lt;</span>
        <span class="comment">// The next two grammar rules are customization points.</span>
        <span class="comment">// The associated transforms are specified externally</span>
        <span class="comment">// using external_transforms below.</span>
        <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span> <span class="identifier">int_terminal</span><span class="special">,</span> <a class="link" href="external_transform.html" title="Struct external_transform">proto::external_transform</a> <span class="special">&gt;</span>
      <span class="special">,</span> <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span> <span class="identifier">char_terminal</span><span class="special">,</span> <a class="link" href="external_transform.html" title="Struct external_transform">proto::external_transform</a> <span class="special">&gt;</span>
      <span class="special">,</span> <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span>
            <a class="link" href="plus.html" title="Struct template plus">proto::plus</a><span class="special">&lt;</span> <span class="identifier">my_grammar</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">&gt;</span>
          <span class="special">,</span> <a class="link" href="fold.html" title="Struct template fold">proto::fold</a><span class="special">&lt;</span> <a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span> <span class="keyword">int</span><span class="special">(</span><span class="special">)</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">&gt;</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>

<span class="comment">// Here is where the transforms are associated with the</span>
<span class="comment">// grammar rules above.</span>
<span class="keyword">struct</span> <span class="identifier">my_transforms</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special">&lt;</span>
        <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span><span class="identifier">int_terminal</span><span class="special">,</span> <span class="identifier">print</span><span class="special">(</span><a class="link" href="_value.html" title="Struct _value">proto::_value</a><span class="special">)</span><span class="special">&gt;</span>
      <span class="special">,</span> <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span><span class="identifier">char_terminal</span><span class="special">,</span> <span class="identifier">print</span><span class="special">(</span><a class="link" href="_value.html" title="Struct _value">proto::_value</a><span class="special">)</span><span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>

<span class="comment">// ...</span>

<a class="link" href="literal.html" title="Struct template literal">proto::literal</a><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">1</span><span class="special">)</span><span class="special">;</span>
<a class="link" href="literal.html" title="Struct template literal">proto::literal</a><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span> <span class="identifier">c</span><span class="special">(</span><span class="char">'a'</span><span class="special">)</span><span class="special">;</span>
<span class="identifier">my_transforms</span> <span class="identifier">trx</span><span class="special">;</span>

<span class="comment">// Evaluate "i+c" using my_grammar with the specified transforms:</span>
<span class="identifier">my_grammar</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">i</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">trx</span><span class="special">)</span><span class="special">;</span>

<span class="comment">// If you would also like to pass arbitrary data along with the</span>
<span class="comment">// transforms, you can use a transform environment, as so:</span>
<span class="identifier">my_grammar</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">i</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span> <span class="special">=</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span> <span class="special">=</span> <span class="identifier">trx</span><span class="special">)</span><span class="special">)</span><span class="special">;</span></pre>
<p>
          </p>
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<td align="right"><div class="copyright-footer">Copyright © 2008 Eric Niebler<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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